The effect of stress on reproduction in snapper (Pagrus auratus)

Stress inhibits reproduction in most fish and can be exerted through normal hatchery practices in aquaculture. Previous research with wild New Zealand snapper, has shown inhibitory effects of stress on reproduction, however, there is no information on the effect of stress in captive or domesticated stocks. This study examines the role of stress in the reproduction of snapper, by determining the effect of capture and confinement on plasma steroid levels, gonadal condition, the ovulatory response to exogenous hormone treatment and in vitro ovarian steroidogenesis. The effects of stress were investigated in wild fish caught by trap, 5-year-old fish, caught as juveniles and on-grown in captivity and 2- and 3-year-old hatchery-reared snapper. Fish were held post capture for up to 168h. Blood was sampled at 0h and after the confinement period, the fish were sacrificed and the gonads preserved for histology. Plasma levels of cortisol and the gonadal steroids, 17˜í‚â§-estradiol (`E_2`), testosterone (T), 11-ketotestosterone (11KT) and 17,20˜í‚⧠dihydroxy-4-pregnen-3-one (17,20˜í‚â§P) were determined by radioimmunoassay (RIA). Stress resulted in an increase in plasma cortisol and concomitant decreases in `E_2`, and T in females, and plasma T and 11 KT in males. In addition there was an increase in the incidence of ovarian atresia in females, and in the proportion of spermatozoa in the testes of males. These results confirm that snapper are highly susceptible to stress-induced impairment of reproduction, and this response is still present in hatchery-reared fish. Induced ovulation is a common requirement in aquaculture, but stress effects associated with handling may affect the efficacy of exogenous hormone treatments. This was tested by treating hatchery-reared female snapper, Oh (unstressed) or 24h after capture (stressed), with either luteinising hormone releasing hormone analogue (LHRHa), human chorionic gonadotropin (hCG), or 17˜í¬±-hydroxyprogesterone (17P). Blood was sampled prior to treatment and again after 168 h and fish were checked periodically for ovulation. In unstressed fish, hCG gave the best ovulatory response, followed by LHRHa, in terms of numbers of ovulators, egg volumes, egg quality and percent fertilisation. A delay in injection resulted in significantly lower `E_2` and T at injection, smaller egg volumes, and poorer egg quality, confirming that treatment at first capture, yields a better ovulatory response than treatment after exposure to capture and handling stress. Studies on plasma levels of gonadal steroids suggest that stress effects of the type found in snapper might result from impairment of ovarian steroidogenesis. Accordingly 3-year-old female hatchery-reared snapper were stressed for up to 168h and isolated ovarian follicles were incubated with hCG, 17P, or hCG plus 17P. 17P was most effective at stimulating production of `E_2` and T, and there was no further benefit in co-stimulation with hCG. Stress markedly reduced the capacity of ovarian follicles to convert 17P to T. `E_2` production was unaffected, suggesting that aromatase-mediated conversion of T to `E_2` is not affected by stress. This indicates that decreases in E2 concentrations evident in the plasma following stress are possibly the result of stress-induced reduction in substrate availability and not a reduction in aromatase activity.